The invention relates to a particle-optical apparatus comprising: a vacuum chamber, an particle-optical column mounted on the vacuum chamber for producing a particle beam round a particle-optical axis, said particle-optical column comprising a particle source, a sample carrier located in the vacuum chamber for carrying a sample at a sample position, said sample position located on the particle-optical axis, a mirror collecting light emitted from the sample position or focussing light on the sample position, said mirror showing a through-hole round the particle-optical axis, and evacuation means for evacuating the apparatus.
The invention also relates to a kit for upgrading an existing apparatus, and to a method for using such an apparatus.
Such an apparatus is known from U.S. Pat. No. 6,885,445 B2.
Such apparatus are used in industry and research, e.g. for the analysis and modification of samples, such as samples taken from semiconductor wafers or e.g. biological samples.
Such an apparatus can be used to irradiate a sample with a beam of particles, such as a beam of electrons or ions. Particle-optical columns producing such beams are known per se from use in Scanning Electron Microscopes (SEM), Focused Ion Beam apparatus (FIB), Scanning Transmission Electron Microscopes (STEM), Electron Microprobes, etc.
As known to the person skilled in the art, images may be obtained by irradiating the sample with a finely focused beam of particles such as electrons, which beam is scanned over the sample. This irradiation causes secondary particles and radiation to emanate from the sample, which particles and radiation can be detected by suitable detectors. As the beam is finely focused and scanned over the sample, the signal detected by the detectors is place dependent. The signal can be converted to an image, e.g. by placing it in a computer memory, after which the data of this memory can be displayed on a display. It is also possible to perform certain analysis on the data in the computer memory and display the outcome of these analysis.
For certain applications it is necessary to irradiate the sample placed in such an apparatus in situ with a beam of light, e.g. to cause heating or to cause chemical changes in the sample. In other cases there is a need to detect light emitted by the sample, e.g. in response to the irradiation with the beam of particles (cathodoluminescence), or in response to irradiation with light (e.g. Raman spectroscopy. FRET (Fluorescent Resonant Energy Transfer), or FLIM (Fluorescent Lifetime IMaging)).
In both cases the sample is irradiated with a particle beam, while a path of light must be present between the sample and e.g. a detector or a generator of light. Also techniques where the sample is simultaneously illuminated by light and photons emanating from the sample are analysed/detected can be envisaged, where e.g. a beam splitter is used to split the path of incoming and outgoing photons.
The known apparatus comprises a particle-optical column showing a particle-optical axis. A sample can be positioned on a sample position at the particle-optical axis. A mirror is placed between the particle-optical column and the sample position. The mirror has a through-hole for passing the particle-beam. Between the mirror the light-optical axis and the particle-optical axis coincide. The mirror, e.g. a parabolic mirror, deflects the light-optical axis over an angle of e.g. 90°, while leaving the particle-optical axis unchanged. As a result e.g. a light-optical detector and/or a generator of light such as a laser, LED or conventional lamp do not interfere with the volume taken up by the particle-optical column and associated detectors, such as a secondary particle detector or an X-ray detector. When using this apparatus for e.g. Raman spectroscopy the numerical aperture of the optical path as defined by the mirror must be large, as the signal is rather weak.
As known to a person skilled in the art, charged particles are easily scattered when travelling through a (rarefied) gas. For that reason the volume through which the charged particles travel in a particle-optical column is normally evacuated to a pressure lower than e.g. 10−4 mbar to avoid scattering.
A problem occurring at such low pressures is that the sample dehydrates, leading to artefacts. Obviously this effect will not take place, at least not to such an extent, when the sample is placed in a vacuum in which water vapour is present. This implies a vacuum with a pressure equal to or exceeding the equilibrium pressure of water vapour.
Instruments are known in which the sample is observed in a vacuum of e.g. 8 mbar, being the equilibrium pressure of water at a temperature of 4° C. Such instruments are commonly known as Environmental Scanning Electron Microscope (ESEM®) or Variable Pressure Scanning Electron Microscope (VP-SEM).
A known limitation of such instruments is that many particles are scattered out of the primary beam of particles into the so-named skirt region. This is described in “Electron Scattering by gas in the scanning electron microscope”, D. A. Moncrieff et al., J. Phys. D: Appl. Phys, Vol. 12 (1979), p 481-488, further referred to as Moncrieff. Moncrieff describes in section 4 (discussion and conclusion) that under the conditions of an electron beam energy of 25 keV and a working distance of 15 mm of nitrogen gas at a pressure of 1 mbar, 60% of the electrons are scattered out of the beam. This implies that only 40% of the particles arrive at the intended position.
FIG. 5 of Moncrieff shows that at a pressure of 10 mbar and a working distance of approximately 4 mm already more than 90% of the particles are scattered. Later work described in “The beam-gas and signal-gas interactions in the variable pressure scanning electron microscope”, C. Mathieu, Scanning Microscopy, Vol. 13, No. 1 (1999), pages 23-44 show that the scattering effect is even more severe at lower beam energies.
“X-ray microanalysis in the variable pressure (environmental) scanning electron microscope”, D. E. Newbury, J. Res. Natl. Inst. Stand. Techn., Vol 107 (2002), pages 567-603, further referred to as Newbury, describes that the signal detected (in this case by an X-ray detector) is the combination of the signal caused by the non-scattered particles and the signal caused by the scattered particles. As the scattered particles can impinge on the sample in the skirt region, removed from the intended position (the focus of the beam of unscattered particles), a signal is generated both by the unscattered particles carrying information about the intended position and by the scattered particles carrying information about the skirt regions. The signal caused by the scattered particles thus ‘pollutes’ the signal caused by the unscattered particles, which may give rise to artefacts, i.e. show the presence of materials at the intended position while these materials are in reality not present at that position, but somewhere in the skirt region. As the fraction of scattered particles rises, the effect becomes stronger.
Although Newbury describes this effect for X-ray microanalysis, it is clear that this effect also occurs when detecting photons coming from the sample, e.g. detecting cathodoluminescense from the sample.
As described in Newbury (page 572, left column), a related problem is the lowering of the signal-to-noise ratio S/N of the signal obtained with e.g. the secondary particle detector. This can be explained as follows: the current density in the unscattered part of the beam is high when compared to the current density in the scattered part. The unscattered particles give a signal with a high spatial resolution, while the scattered particles give a signal with a much lower spatial resolution, as the skirt region is much larger. Therefore, even when most of the particles are scattered, the place dependency of the combined signal is sufficient to obtain resolution. However, as less and less particles are available in the unscattered part of the beam, the signal caused by this part of the beam will drop accordingly. A lowering of the S/N is thus the effect.
The effect of scattering is not discussed in U.S. Pat. No. 6,885,445 B2.
Practical marketed systems for performing e.g. Raman spectroscopy, Fluorescent Microscopy (FM), Fluorescent Resonant Energy Transfer (FRET) or Fluorescent Lifetime IMaging (FLIM) in conjunction with an ESEM® typically use a distance of approximately 10 mm between the mirror and the sample position due to the minimum practical size of the mirror while obtaining a large collection angle (a high numerical aperture N.A. of e.g. N.A.>0.24) for light. At a pressure of 8 mbar more than 90% of the particle of a 25 kV particle beam would thus be scattered out of the beam into the skirt region (see Moncrieff, FIG. 5). Most of the signal detected is thus coming from the skirt region, and not from the intended focus. This would lead to a large deterioration of the S/N ratio of the signal obtained by the particle-optical column, and would also lead to most of the signal coming from the skirt region on the sample instead of from the intended region: the focus of the unscattered beam
A disadvantage of such a system is thus the problems associated with the scattering of particles from the particle beam at elevated pressures of e.g. 8 mbar.
There is thus a need for a particle-optical apparatus of the kind disclosed in U.S. Pat. No. 6,885,445 B2, showing a lower scattering of the particles of the particle beam at elevated pressures.
It is an object of the invention to provide such an apparatus.